Hologram recording material, process for producing the same and hologram recording medium

ABSTRACT

The present invention provides a hologram recording material which is suitable for volume hologram record and attains high refractive index change, flexibility, high sensitivity, low scattering, environment resistance, durability, low shrinkage and high multiplicity in holographic memory record using not only a green laser but also a blue laser; a process for producing the same; and a hologram recording medium having the hologram recording material. A hologram recording material comprising: a metal oxide containing at least Si and Zr as metals, wherein an aromatic carboxylic acid compound is coordinated to Zr; and a photopolymerizable compound. The aromatic carboxylic acid compound is, for example, toluic acid. A hologram recording medium ( 11 ) comprising the hologram recording material layer ( 21 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hologram recording material suitablefor volume hologram recording, a process for producing the same, and ahologram recording medium having the hologram recording material. Thepresent invention relates in particular to a hologram recording materialsuitable for record and reproduction using not only a green laser lightbut also a blue laser light, a process for producing the same, and ahologram recording medium having the hologram recording material.

2. Disclosure of the Related Art

Research and development of holographic memories have been advanced aslarge-capacity recording technique making high-speed transmissionpossible. O plus E, vol. 25, No. 4, 385-390 (2003) describes basicstructures of holographic memories and a coming prospect thereof.

Examples of the property required for a hologram recording materialinclude high refractive index change at the time of recording, highsensitivity, low scattering, environment resistance, durability, lowdimensional change, and high multiplicity. About holographic memoryrecord using a green laser, various reports have been made hitherto asfollows.

As a hologram recording material, there is known a photopolymer materialmade mainly of an organic binder polymer and a photopolymerizablemonomer. However, the photopolymer material has problems aboutenvironment resistance, durability and others. In order to solve theproblems of the photopolymer material, attention has been paid to anorganic-inorganic hybrid material made mainly of an inorganic matrix anda photopolymerizable monomer, and the hybrid material has beeninvestigated. The inorganic matrix is excellent in environmentresistance and durability.

For example, Japanese Patent No. 2953200 discloses a film for opticalrecording wherein a photopolymerizable monomer or oligomer and aphotopolymerization initiator are contained in an inorganic substancenetwork film. It is also disclosed that the brittleness of the inorganicnetwork film is improved by modifying the inorganic network organically.However, the compatibility between the inorganic substance network andthe photopolymerizable monomer or oligomer is bad. Therefore, a uniformfilm is not easily obtained. A specific disclosure of the publication isthat a photosensitive layer having a thickness of about 10 μm (par.[0058]) is exposed to an argon laser having a wavelength of 514.5 nm(par. [0059]).

JP-A-11-344917 discloses an optical recording medium wherein anorganic-inorganic hybrid matrix contains an optically active monomer. Inthe organic-inorganic hybrid matrix, a metal element has an alkyl group(a methyl group) or an aryl group (a phenyl group). However, theintroduction of the methyl group makes it impossible to improve thecompatibility between the hybrid matrix and the optically activemonomer. The introduction of the phenyl group gives a more improvementin the compatibility than the introduction of the methyl group. However,the introduction of the phenyl group causes a fall in the curing speedof a hybrid matrix precursor ([0015] in the above publication). Aspecific disclosure of the publication is that record is made in ahologram recording layer having a thickness of 100 μm, using a YAG laserhaving a wavelength of 532 nm (Example, [0031]).

JP-A-2002-236439 discloses a hologram recording material comprising: amatrix made of an organic-inorganic hybrid polymer obtained bycopolymerizing an organometallic compound containing an ethylenicallyunsaturated double bond and an organic monomer having an ethylenicallyunsaturated double bond, as main chain constituting components, and/or ahydrolyzed polycondensate thereof; a photopolymerizable compound; and aphotopolymerization initiator. By the introduction of the large organicmain chain component into the matrix material, the compatibility betweenthe matrix and the photopolymerizable compound is improved. However, theintroduction of the large organic main chain component permits thepresence of a two-component structure of the organic main chain and aninorganic network in the matrix material. Thus, it appears that thematrix may not exhibit unified behavior at the time of recording so asto cause nonuniform recording. If the ratio of the organic main chaincomponent in the matrix is large, the same problems as in the case ofthe above-mentioned photopolymer material, which uses an organic binderpolymer, are caused. A specific disclosure of the publication is that ahologram recording material layer having a thickness of 20 μm (par.[0080]) is exposed to an argon laser having a wavelength of 514.5 nm(par. [0081]).

In order to solve the problems of the hologram recording materialsdisclosed in the above-mentioned individual publications,JP-A-2005-321674 discloses a hologram recording material comprising: anorganometallic compound at least containing at least two kinds of metals(Si and Ti), oxygen, and an aromatic group, and having an organometallicunit wherein two aromatic groups are directly bonded to one metal (Si);and a photopolymerizable compound. In Example 1 of the publication (inparticular, pars. [0074] to [0078]), it is disclosed that a hologramrecording medium which has a layer of the above-mentioned hologramrecording material having a thickness of 100 μm gave a hightransmittance, a high refractive index change, a low scattering, and ahigh multiplicity in record using a Nd:YAG laser (532 nm).

SUMMARY OF THE INVENTION

Any of the above-mentioned publications disclose holographic memoryrecord using a green laser, but do not disclose holographic memoryrecord using a blue laser.

An object of the present invention is to provide a hologram recordingmaterial which is suitable for volume hologram record and can attainhigh refractive index change, flexibility, high sensitivity, lowscattering, environment resistance, durability, low dimensional change(low shrinkage) and high multiplicity in holographic memory record usingnot only a green laser but also a blue laser; and to provide a processfor producing the a hologram recording material. And, an object of thepresent invention is to provide a hologram recording medium which has ahologram recording layer comprising the hologram recording material.

The present inventors have made investigations, so as to find out thatwhen a blue laser is used to make a holographic memory record in thehologram recording medium disclosed in JP-A-2005-321674, thetransmittance thereof falls so that good holographic memory recordingcharacteristics cannot be obtained. It has also been understood that thefall in the transmittance results from a matter that the recording layerof the medium contains Ti as a constituent metallic element thereof.When a transmittance falls, holograms (interference fringes) areunevenly formed in the recording layer along the thickness direction ofthe recording layer so that scattering-based noises and the like aregenerated. It has been found out that in order to obtain good hologramimage characteristics, it is necessary that the medium has a lighttransmittance of 50% or more.

A light transmittance of a hologram recording layer depends on athickness thereof. As the thickness of the recording layer is madesmaller, the light transmittance is improved; however, the widths ofdiffraction peaks obtained when reproducing light is irradiated into arecorded pattern become larger so that separability between adjacentdiffraction peaks deteriorates. Accordingly, in order to obtain asufficient SN ratio (Signal to Noise ratio), it is indispensable to makea shift interval (an angle or the like) large when multiple record ismade. For this reason, a high multiplicity cannot be attained. In theuse of a hologram recording medium in any recording system, thethickness of its recording layer is required to be at lowest 100 μm inorder to attain holographic memory recording characteristics forensuring a high multiplicity.

The present invention includes the followings:

-   (1) A hologram recording material comprising:

a metal oxide containing at least Si and Zr as metals, wherein anaromatic carboxylic acid compound is coordinated to Zr; and

a photopolymerizable compound.

-   (2) The hologram recording material according to the above-described    (1), wherein the aromatic carboxylic acid compound is toluic acid.-   (3) The hologram recording material according to the    above-described (1) or (2), wherein the metal oxide has an    organometallic unit in which two aromatic groups are directly bonded    to Si.-   (4) The hologram recording material according to any one of the    above-described (1) to (3), which further comprises a    photopolymerization initiator.-   (5) A process for producing a hologram recording material    comprising:

a metal oxide containing at least Si and Zr as metals, wherein anaromatic carboxylic acid compound is coordinated to Zr; and

a photopolymerizable compound,

the process comprising:

mixing an alkoxide compound of Zr as a starting material with anaromatic carboxylic acid compound to modify the alkoxide compound of Zrwith the aromatic carboxylic acid compound;

mixing the alkoxide compound of Zr modified with the aromatic carboxylicacid compound with an alkoxide compound of Si;

hydrolyzing the mixed alkoxide compounds to yield a precursor of a metaloxide;

incorporating a photopolymerizable compound into the resultant systembefore, during or after the hydrolysis; and

advancing polycondensation reaction of the metal oxide precursor intowhich the photopolymerizable compound is incorporated.

-   (6) A hologram recording medium which has a hologram recording layer    comprising the hologram recording material according to any one of    the above-described (1) to (4).-   (7) The hologram recording medium according to the above-described    (6), wherein record/reproduction of said hologram recording medium    are performed using a laser light having a wavelength of 350 to 450    nm.-   (8) The hologram recording medium according to the    above-described (6) or (7), wherein said hologram recording medium    has a light transmittance of 50% or more at a wavelength of 405 nm,    or has a light reflectance of 25% or more at a wavelength of 405 nm.-   (9) The hologram recording medium according to any one of the    above-described (6) to (8), wherein the hologram recording layer has    a thickness of at least 0.1 mm (100 μm).-   (10) An alkoxide compound of Zr, wherein an aromatic carboxylic acid    is coordinated to Zr.

According to the hologram recording material of the present invention,the organic group-containing metal oxide, which functions as a matrix ora dispersing medium for the photopolymerizable compound, contains Si andZr as constituent metallic elements, wherein an aromatic carboxylic acidcompound is coordinated to Zr; therefore, the material is in a very even(or homogeneous) form, and less absorbs light having a wavelength in theblue wavelength resion than materials containing Ti as a constituentmetallic element.

For this reason, using the hologram recording material of the presentinvention, provided is a hologram recording medium which has a higherlight transmissivity to a blue laser and gives a high sensitivity and ahigh multiplicity in recording/reproducing using not only a green laserlight but also a blue laser light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a schematic cross section of a hologramrecording medium produced in the example.

FIG. 2 is a plane view illustrating the outline of a hologram recordingoptical system used in the example.

DETAILED DESCRIPTION OF THE INVENTION

The hologram recording material of the present invention is acomposition containing the following as essential components: an organicgroup-containing metal oxide (namely, organic-inorganic hybrid)containing at least Si and Zr as metals, wherein an aromatic carboxylicacid compound as a complexing ligand is coordinated to Zr; and aphotopolymerizable compound (namely, a monomer). The hologram recordingmaterial is made into a film form, thereby preparing a hologramrecording layer. Any optional metal other than Si and Zr may be furtherused. However, it is preferred not to use Ti. The incorporation of thetwo or more kinds of metals as constituent elements into the materialmakes the control of characteristics of the material, such as therefractive index, easy. This is preferred for the design of therecording material. In the present specification, a hologram recordinglayer may be called a hologram recording material layer.

The organic group-containing metal oxide is obtained by subjecting analkoxide compound of Zr which is modified with an aromatic carboxylicacid compound as a complexing ligand and a corresponding alkoxidecompound of Si to hydrolysis and polymerization reaction (the so-calledsol-gel reaction). The metal oxide is in a very even gel or sol form.The organic group-containing metal oxide functions as a matrix or adispersing medium for the photopolymerizable compound in the hologramrecording material layer. In other words, the photopolymerizablecompound in a liquid phase is evenly dispersed with a good compatibilityin the organic group-containing metal oxide in a gel- or sol-form.

When light having coherency is irradiated onto the hologram recordingmaterial layer, the photopolymerizable organic compound (monomer)undergoes polymerization reaction in the exposed portion so as to bepolymerized, and further the photopolymerizable organic compounddiffuses and shifts from the unexposed portion into the exposed portionso that the polymerization of the exposed portion further advances. As aresult, an area where the polymer produced from the photopolymerizableorganic compound is large in amount and an area where the polymer issmall in amount are formed in accordance with the intensity distributionof the light. At this time, the organic group-containing metal oxideshifts from the area where the polymer is large in amount to the areawhere the polymer is small in amount, so that the area where the polymeris large in amount becomes an area where the metal oxide is small inamount and the area where the polymer is small in amount becomes an areawhere the metal oxide is large in amount. In this way, the lightexposure causes the formation of the area where the polymer is large inamount and the area where the metal oxide is large in amount. When arefractive index difference exists between the polymer and the metaloxide, a refractive index change is recorded in accordance with thelight intensity distribution.

In order to obtain a better recording property in the hologram recordingmaterial, it is necessary that a difference is large between therefractive index of the polymer produced from the photopolymerizablecompound and that of the metal oxide. The refractive indices of thepolymer and the metal oxide may be designed so as to make any one of therefractive indices high (or low).

In the case of using a large amount of Si, a design for making arefractive index of the metal oxide low is supposed. In the presentinvention, however, the metal oxide contains Zr as essential constituentelement thereof, therefore, a high refractive index of the metal oxidecan be obtained. Accordingly, it is advisable to design the hologramrecording material so as to cause the metal oxide to have a highrefractive index and cause the polymer to have a low refractive index.

It is advisable to determine the number of Si atoms and that of Zr atomsin the metal oxide appropriately, considering a desired refractiveindex. For example, the number (s) of the Si atoms, and the number (m)of the Zr atoms and any other optional metal atoms (such as Ta, Ge, Sn,Al and Zn) in the oxide preferably satisfy the following relationship:

0.3 s≦m≦3 s.

If the number (m) of the Zr atoms and any other optional metal atoms isless than 0.3 s, the effect based on the incorporation of the two ormore kinds of metals into the metal oxide, that is, the effect that therefractive index or other properties are easily controlled becomessmall. On the other hand, if the number (m) is larger than 3 s, themetal oxide is liable to have the nature of an inorganic matrix, as awhole of the oxide, so that the compatibility with organic matters orflexibility thereof lowers.

In the present invention, an aromatic carboxylic acid compound as acomplexing ligand is coordinated to at least one portion of Zr atomswhich constitutes the metal oxide. The aromatic carboxylic acid is acompound wherein one or more carboxylic acid group(s) (—COOH) are bondeddirectly to an aromatic ring (Ar). Examples of the aromatic carboxylicacid include aromatic monocarboxylic acids such as benzoic acid, o-, m-or p-toluic acid, and o-, m- or p-methoxybenzoic acid; and aromaticdicarboxylic acids such as phthalic acid, and terephthalic acid.

When a mixture of the alkoxide compound of Zr and the alkoxide compoundof Si is subjected to a sol-gel reaction, the alkoxide compound of Si isgenerally small in rates of hydrolysis and polymerization reaction andthe alkoxide compound of Zr is large in rates of hydrolysis andpolymerization reaction. As a result, an oxide of Zr aggregates so thata homogeneous sol-gel reaction product cannot be obtained. The presentinventors have made investigations to find out that in the case ofmodifying an alkoxide compound of Zr chemically with an aromaticcarboxylic acid compound by coordinating the aromatic carboxylic acidcompound to the Zr alkoxide compound before the sol-gel reaction, thehydrolysis and polymerization reaction thereof can be appropriatelyrestrained (or controlled) to yield a homogeneous sol-gel reactionproduct from a mixture of the Zr alkoxide compound with an alkoxidecompound of Si.

It appears that: the aromatic carboxylic acid is coordinated to a Zralkoxide compound Zr(OR)₄ wherein R's are each an alkyl group, so thatthe molecules of the Zr alkoxide compound change into a clustercontaining several Zr atoms, or Zr alkoxide compounds such asZr(OR)₃(HOOC—Ar), Zr(OR)₂(HOOC—Ar)₂, Zr(OR) (HOOC—Ar)₃; or the aromaticcarboxylic acid may be coordinated to Zr atoms in two molecules of theZr alkoxide compound so as to extend onto the Zr atoms. As a result, thehydrolysis and polymerization reaction would be restrained by areduction in the number of the alkoxy groups that can contribute to thehydrolysis and polymerization reaction, and by restraint of thereactivity of the alkoxy groups based on a steric factor of the aromaticcarboxylic acid compound. In such a manner, the organic group-containingmetal oxide in the present invention is in a very homogeneous gel or solform. The organic group-containing metal oxide less absorbs light havinga wavelength in the blue wavelength region than metal oxide containingTi as a constituent metal element thereof.

The amount of the aromatic carboxylic acid compound to be used is notparticularly limited, and is appropriately determined on the basis ofthe amount of the Zr alkoxide compound, considering the above-mentionedreaction restraining effect. For example, in the case of an aromaticmonocarboxylic acid compound, it is advisable to use the aromaticcarboxylic acid to set the ratio by mole of the aromatic carboxylic acidcompound to the Zr atoms in the used Zr alkoxide compound (the aromaticmonocarboxylic acid compound/the Zr atoms) into the range of not lessthan 0.8/1 to not more than 3/1, for example, not less than 1/1 to notmore than 2.5/1, preferably not less than 1.5/1 to not more than 2/1. Ifthe mole ratio of the aromatic monocarboxylic acid compound/the Zr atomsis less than 0.8/1, the amount of the aromatic monocarboxylic acidcompound is small; thus, the rate of the hydrolysis and polymerizationreaction of the Zr alkoxide compound is large so that a homogeneoussol-gel reaction product is not easily obtained in the coexistence ofthe Si alkoxide compound. As a result, a high sensitivity and a highmultiplicity of the hologram recording medium are not easily attained.If the above-mentioned mole ratio is more than 3/1, the amount of thearomatic monocarboxylic acid is excessive; thus, the ratio of thehydrolysis and polymerization reaction of the Zr alkoxide compound areexcessively restrained so that the curing of the sol-gel reactionproduct does not advance easily.

The Zr alkoxide compound as the starting material is not limited, but ispreferably a Zr alkoxide compound represented by:

Zr(OR)₄   (I)

wherein R's are each a linear saturated alkyl group having 3 to 4 carbonatoms. Four R's may be the same or different. Specific examples thereofinclude tetra-n-propoxyzirconium, and tetra-n-butoxyzirconium. The Zralkoxide compound, which has a linear saturated alkoxide group, ispreferred since the compound is smaller in steric hindrance than Zralkoxide compounds having a branched alkoxide group so that the aromaticcarboxylic acid compound is more easily coordinated thereto. However,tetra-iso-propoxyzirconium, tetra-tert-butoxyzirconium or the like maybe used.

As the alkoxide compound of Si, an oligomer which is a partiallyhydrolytic condensate of said Si alkoxide compound may be used. As thealkoxide compound of Zr, an oligomer which is a partially hydrolyticcondensate of said Zr alkoxide compound may be used.

In the organic group-containing metal oxide, the metal atoms (the samemetal atoms and the different metal atoms) are bonded to each otherthrough an oxygen atom interposed therebetween. For the purposes such asto obtain a desired refractive index in the present invention, the metaloxide may contain any optional metal(s) other than Si and Zr, forexample, metal(s) selected arbitrarily from Ta, Ge, Sn, Al and Zn, insuch an amount that the high sensitivity and the high multiplicity ofthe hologram recording medium are not damaged.

In the present invention, a preferred organic group-containing metaloxide which has flexibility will be described hereinafter.

In the present invention, in order to obtain a much better recordingproperty in the hologram recording material, it is necessary that thediffusion/polymerization of the photopolymerizable compound is easilyattained in the state that the photopolymerizable compound is presenttogether with the organic group-containing metal oxide. If the organicgroup-containing metal oxide has flexibility, it functions as a matrixor a dispersing medium for the photopolymerizable compound so that thediffusion/polymerization of the photopolymerizable compound is easilyattained. Thus, the refractive index change between the exposed portionand the unexposed portion becomes larger by irradiation of light.

In the present invention, the preferred organic group-containing metaloxide contains Si and Zr as metals (M), contains an aromatic carboxylicacid compound coordinated to Zr, and further contains an organometallicunit (Ar—Si—Ar), wherein two aromatic groups (Ar) are directly bonded toSi. Alternatively, the metal oxide may further contain, as metals (M),any optional metal other than Si and Zr (provided that Ti is notcontained), and further contains an organometallic unit (Ar-M-Ar),wherein two aromatic groups (Ar) are directly bonded to the optionalmetal (M). The metal oxide having such an organometallic unit hasflexibility.

In order to introduce the organometallic unit (Ar-M-Ar) into the metaloxide, it is advisable to use a diarylalkoxide compound of the metal(M), which constitutes the organometallic unit, in the sol-gel reaction.

The diarylalkoxide compound of Si as a starting material is available atease. Examples of the diarylalkoxide compound of Si includediphenyldimethoxysilane, and diphenyldiethoxysilane. However, it is notexcluded that aromatic groups are bonded directly to the metal otherthan Si.

It is more preferred that the organometallic unit (Ar—Si—Ar) is a unit(Ph-Si-Ph) wherein two phenyl groups (Ph) are bonded directly to one Siatom. The diphneylalkoxide compound of Si as a starting material isavailable at ease, and the compound is good in reactivity for hydrolysisand polymerization. The phenyl groups may have a substituent.

The preferred organic group-containing metal oxide has an organometallicunit wherein two aromatic groups are bonded directly to one metal.Besides the organometallic unit, the organometallic compound may have anorganometallic unit wherein one aromatic group is bonded directly to onemetal, or may have an organometallic unit wherein three aromatic groupsare bonded directly to one metal.

According to the introduction of the two aromatic groups (phenyl groups)into Si, the organometallic compound has good compatibility with thephotopolymerizable compound described below and an organic polymerproduced by the polymerization thereof. The refractive index of theorganic group-containing metal oxide also becomes high.

In the present invention, it is preferred that the number (p) of thephenyl group, and the number (s) of the Si atom which are contained inthe organic group-containing metal oxide satisfy the followingrelationship in the above-mentioned metal oxide composition:

s≦p<3 s.

Specifically, it is preferred that one or more and less than threephenyl groups are bonded to one Si atom, as the whole of the metal oxidecomposition, from the viewpoint of the compatibility with thephotopolymerizable compound and an organic polymer produced by thepolymerization thereof.

In the present invention, an organic group other than the aromaticgroups, for example, an alkyl group may be introduced into Si in themetal oxide. For example, methylphenyldimethoxysilane or the like can beused as long as the advantageous effects of the present invention arenot damaged. When a monoalkoxysilane such as trimethylmethoxysilane ispresent, the polymerization reaction is terminated. Accordingly, themonoalkoxysilane can be used to adjust the molecular weight.

The metal oxide may contain trace amounts of elements other than theabove.

In the present invention, the photopolymerizable compound is aphotopolymerizable monomer. As the photopolymerizable compound, acompound selected from a radical polymerizable compound and a cationpolymerizable compound can be used.

The radical polymerizable compound is not particularly limited as longas the compound has in the molecule one or more radical polymerizableunsaturated double bonds. For example, a monofunctional andmultifunctional compound having a (meth)acryloyl group or a vinyl groupcan be used. The wording “(meth)acryloyl group” is a wording forexpressing a methacryloyl group and an acryloyl group collectively.

Examples of the compound having a (meth)acryloyl group, out of theradical polymerizable compounds, include monofunctional (meth)acrylatessuch as phenoxyethyl(meth)acrylate, 2-methoxyethyl(meth)acrylate,2-hydroxyethyl(meth)acrylate, benzyl(meth)acrylate,cyclohexyl(meth)acrylate, ethoxydiethylene glycol(meth)acrylate,methoxypolyethylene glycol(meth)acrylate, methyl(meth)acrylate,polyethylene glycol(meth)acrylate, polypropylene glycol(meth)acrylate,and stearyl(meth)acrylate; and

polyfunctional (meth)acrylates such as trimethylolpropanetri(meth)acrylate, pentaerythritol tetra(meth)acrylate,dipentaerythritol hexa(meth)acrylate, diethylene glycoldi(meth)acrylate, triethylene glycol di(meth)acrylate, tetraethyleneglycol di(meth)acrylate, polyethylene glycol di(meth)acrylate,bis(2-hydroxyethyl)isocyanurate di(meth)acrylate, and2,2-bis[4-(acryloxy-diethoxy)phenyl]propane. However, the compoundhaving a (meth)acryloyl group is not necessarily limited thereto.

Examples of the compound having a vinyl group include monofunctionalvinyl compounds such as monovinylbenzene (styrene), and ethylene glycolmonovinyl ether; and polyfunctional vinyl compounds such asdivinylbenzene, ethylene glycol divinyl ether, diethylene glycol divinylether, and triethylene glycol divinyl ether. However, the compoundhaving a vinyl group is not necessarily limited thereto.

One kind of the radical polymerizable compound may be used, and two ormore kinds thereof are used together. In the case of making therefractive index of the metal oxide high and making the refractive indexof the organic polymer low, in the present invention, a compound havingno aromatic group to have low refractive index (for example, refractiveindex of 1.5 or less) is preferred out of the above-mentioned radicalpolymerizable compounds. In order to make the compatibility with themetal oxide better, preferred is a more hydrophilic glycol derivativesuch as polyethylene glycol(meth)acrylate and polyethylene glycoldi(meth)acrylate.

The cation polymerizable compound is not particularly limited about thestructure as long as the compound has at least one reactive groupselected from a cyclic ether group and a vinyl ether group.

Examples of the compound having a cyclic ether group out of such cationpolymerizable compounds include compounds having an epoxy group, analicyclic epoxy group or an oxetanyl group.

Specific examples of the compound having an epoxy group includemonofunctional epoxy compounds such as 1,2-epoxyhexadecane, and2-ethylhexyldiglycol glycidyl ether; and polyfunctional epoxy compoundssuch as bisphenol A diglycidyl ether, novolak type epoxy resins,trisphenolmethane triglycidyl ether, 1,4-butanediol diglycidyl ether,1,6-hexanediol diglycidyl ether, glycerin triglycidyl ether,trimethylolpropane triglycidyl ether, propylene glycol diglycidyl ether,and polyethylene glycol diglycidyl ether.

Specific examples of the compound having an alicyclic epoxy groupinclude monofunctional compounds such as 1,2-epoxy-4-vinylcyclohexane,D-2,2,6-trimethyl-2,3-epoxybicyclo[3,1,1]heptane, and3,4-epoxycyclohexylmethyl(meth)acrylate; and polyfunctional compoundssuch as 2,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate,bis(3,4-epoxycyclohexylmethyl) adipate,2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxy)cyclohexanone-m-dioxane,bis(2,3-epoxycyclopentyl)ether, and EHPE-3150 (alicyclic epoxy resin,manufactured by Dicel Chemical Industries, Ltd.).

Specific examples of the compound having an oxetanyl group includemonofunctional oxetanyl compounds such as3-ethyl-3-hydroxymethyloxetane,3-ethyl-3-(2-ethylhexyloxymethyl)oxetane, and3-ethyl-3-(cyclohexyloxymethyl)oxetane; and polyfunctional oxetanylcompounds such as 1,4-bis[(3-ethyl-3-oxetanylmethoxy)methyl]benzene,1,3-bis[(3-ethyl-3-oxetanylmethoxy)methyl]propane, ethylene glycolbis(3-ethyl-3-oxetanylmethyl)ether,trimethylolpropanetris(3-ethyl-3-oxetanylmethyl)ether, pentaerythritoltetrakis(3-ethyl-3-oxetanylmethyl)ether, dipentaerythritolhexakis(3-ethyl-3-oxetanylmethyl)ether, and ethylene oxide modifiedbisphenol A bis(3-ethyl-3-oxetanylmethyl)ether.

Specific examples of the compound having a vinyl ether group, out of theabove-mentioned cation polymerizable compounds, include monofunctionalcompounds such as triethylene glycol monovinyl ether,cyclohexanedimethanol monovinyl ether, and 4-hydroxycyclohexyl vinylether; and polyfunctional compounds such as triethylene glycol divinylether, tetraethylene glycol divinyl ether, trimethylolpropane trivinylether, cyclohexane-1,4-dimethylol divinyl ether, 1,4-butanediol divinylether, polyester divinyl ether, and polyurethane polyvinyl ether.

One kind of the cation polymerizable compound may be used, or two ormore kinds thereof may be used together. As the photopolymerizablecompound, an oligomer of the cation polymerizable compounds exemplifiedabove may be used. In the case of making the refractive index of themetal oxide high and making the refractive index of the organic polymerlow, in the present invention, a compound having no aromatic group tohave low refractive index (for example, refractive index of 1.5 or less)is preferred out of the above-mentioned cation polymerizable compounds.In order to make the compatibility with the metal oxide better,preferred is a more hydrophilic glycol derivative such as polyethyleneglycol diglycidyl ether.

It is advisable that in the present invention the photopolymerizablecompound is used, for example, in an amount of about 5 to 1,000% byweight of total of the metal oxide, preferably in an amount of 10 to300% by weight thereof. If the amount is less than 5% by weight, a largerefractive index change is not easily obtained at the time of recording.If the amount is more than 1,000% by weight, a large refractive indexchange is not easily obtained, either, at the time of recording.

In the present invention, it is preferred that the hologram recordingmaterial further contains a photopolymerization initiator correspondingto the wavelength of recording light. When the photopolymerizationinitiator is contained in the hologram recording material, thepolymerization of the photopolymerizable compound is promoted by thelight exposure at the time of recording. Consequently, a highersensitivity is achieved.

When a radical polymerizable compound is used as the photopolymerizablecompound, a photo radical initiator is used. On the other hand, when acation polymerizable compound is used as the photopolymerizablecompound, a photo cation initiator is used.

Examples of the photo radical initiator include Darocure 1173, Irgacure784, Irgacure 651, Irgacure 184 and Irgacure 907 (each manufactured byCiba Specialty Chemicals Inc.). The content of the photo radicalinitiator is, for example, about 0.1 to 10% by weight, preferably about0.5 to 5% by weight on the basis of the radical polymerizable compound.

As the photo cation initiator, for example, an onium salt such as adiazonium salt, a sulfonium salt, or a iodonium salt can be used. It isparticularly preferred to use an aromatic onium salt. Besides, aniron-arene complex such as a ferrocene derivative, anarylsilanol-aluminum complex, or the like can be preferably used. It isadvisable to select an appropriate initiator from these. Specificexamples of the photo cation initiator include Cyracure UVI-6970,Cyracure UVI-6974 and Cyracure UVI-6990 (each manufactured by DowChemical Co. in USA), Irgacure 264 and Irgacure 250 (each manufacturedby Ciba Specialty Chemicals Inc.), and CIT-1682 (manufactured by NipponSoda Co., Ltd.). The content of the photo cation initiator is, forexample, about 0.1 to 10% by weight, preferably about 0.5 to 5% byweight on the basis of the cation polymerizable compound.

The hologram recording material composition may contain a dye thatfunctions as a photosensitizer corresponding to the wavelength ofrecording light or the like besides the photopolymerization initiator.Examples of the photosensitizer include thioxanthones such asthioxanthen-9-one, and 2,4-diethyl-9H-thioxanthen-9-one; xanthenes;cyanines; melocyanines; thiazines; acridines; anthraquinones; andsqualiriums. It is advisable to set a amount to be used of thephotosensitizer into the range of about 3 to about 50% by weight of theradical photoinitiator, for example, about 5% by weight thereof.

A process for producing the hologram recording material will bedescribed in the following.

The hologram recording material of the present invention, the productionprocess of which is not particularly limited, is preferably produced bya production process comprising:

mixing an alkoxide compound of Zr as a starting material with anaromatic carboxylic acid compound to modify the alkoxide compound of Zrwith the aromatic carboxylic acid compound;

mixing the alkoxide compound of Zr modified with the aromatic carboxylicacid compound with an alkoxide compound of Si;

hydrolyzing the mixed alkoxide compounds to yield a precursor of a metaloxide;

incorporating a photopolymerizable compound into the resultant systembefore, during or after the hydrolysis; and

advancing polycondensation reaction of the metal oxide precursor intowhich the photopolymerizable compound is incorporated.

First, an alkoxide compound of Zr as a starting material is mixed withan aromatic carboxylic acid compound to modify the alkoxide compound ofZr with the aromatic carboxylic acid compound. The amount of thearomatic carboxylic acid compound is appropriately determined,considering the above-mentioned reaction restraining effect. As the Zralkoxide compound as the starting material, a Zr alkoxide compoundrepresented by the above-mentioned formula (I) is preferably used.Alternatively, an oligomer which is a partially condensate of said Zralkoxide compound may be used. The solvent used at this time may be thesame solvent as used in the sol-gel reaction.

Next, the Zr alkoxide compound modified with the aromatic carboxylicacid compound, the reactivity of which is restrained by the modificationwith the aromatic carboxylic acid compound, is mixed with an alkoxidecompound of Si. As the Si alkoxide compound, a diarylalkoxide compoundof Si is preferably used. Alternatively, an oligomer which is apartially condensate of said Si alkoxide compound may be used.

Next, the mixed alkoxide compounds are hydrolyzed and polycondensed toyield a precursor of a metal oxide. The hydrolysis and polycondensationreaction can be carried out by the same operation under the sameconditions as in known sol-gel methods. For example, the metal alkoxidecompounds (the diphenylalkoxide compound of Si, and the modifiedcompound of Zr) in a predetermined ratio are dissolved into anappropriate good solvent to prepare a homogeneous solution. Anappropriate acid catalyst is dropwise added to the solution, and thesolution is stirred in the presence of water, whereby the reaction canbe conducted.

Examples of such a solvent include: water; alcohols such as methanol,ethanol, propanol, isopropanol, and butanol; ethers such as diethylether, dioxane, dimethoxyethane and tetrahydrofuran; andN-methylpyrrolidone, acetonitrile, N,N-dimethylformamide,N,N-dimethylacetamide, dimethylsulfoxide, acetone, benzene, and thelike. The solvent may be appropriately selected from these.Alternatively, a mixture of these may be used. The amount of the solventis not limited, and is preferably 10 to 1,000 parts by weight withrespect to 100 parts by weight of the whole of the metal alkoxidecompound.

Examples of the acid catalyst include: inorganic acids such ashydrochloric acid, sulfuric acid, nitric acid and phosphoric acid;organic acids such as formic acid, acetic acid, trichloroacetic acid,trifluoroacetic acid, propionic acid, methanesulfonic acid,ethanesulfonic acid, and p-toluenesulfonic acid; and the like.

The hydrolysis polymerization reaction can be generally conducted atroom temperature, which depends on the reactivity of the metal alkoxidecompounds. The reaction can be conducted at a temperature of about 0 to150° C., preferably at a temperature of about room temperature to 50° C.The reaction time may be appropriately determined, correspondingly tothe relationship with the reaction temperature. The time is about 0.1 to240 hours. The reaction may be conducted in an inert atmosphere such asnitrogen gas, or may be conducted under a reduced pressure of about 0.5to 1 atom while the alcohol produced by the polymerization reaction isremoved.

Before, during or after the hydrolysis, the photopolymerizable organiccompound is mixed. The photopolymerizable organic compound may be mixedwith the metal alkoxide compounds as the sol-gel starting materialsafter, during or before the hydrolysis. In the case of the mixing afterthe hydrolysis, it is preferred to add and mix the photopolymerizableorganic compound in the state that the sol-gel reaction systemcontaining the metal oxide and/or the metal oxide precursor is sol inorder to perform the mixing uniformly. In the case of the mixing beforethe hydrolysis, the mixing of the photopolymerizable organic compound ispreferably conducted after the modification step of the Zr alkoxidecompound, and after the mixing step of the modified Zr alkoxide compoundand the Si alkoxide compound. The mixing of a photopolymerizationinitiator or photosensitizer can also be conducted before, during orafter the hydrolysis.

A polycondensation reaction of the metal oxide precursor with which thephotopolymerizable compound is mixed is advanced to yield a hologramrecording material liquid wherein the photopolymerizable compound isuniformly incorporated in a sol-form metal oxide matrix. This hologramrecording material liquid contains the metal oxides that have variousmolecular weights.

In this process, the reactivity of the Zr alkoxide compound iscontrolled or restrained by modifying said compound with the aromaticcarboxylic acid compound. Therefore, the organic group-containing metaloxide matrix is obtained in a very even gel- or sol-form.

The hologram recording material liquid in which the photopolymerizablecompound and the sol-form organic group-containing metal oxide areuniformly mixed with each other is applied onto a substrate, and thenthe solvent is dried. As the case may be, the sol-gel reaction isfurther advanced. As a result, a hologram recording material layer in afilm form is yielded. In such a way, the hologram recording materiallayer is produced wherein the photopolymerizable organic compound isuniformly incorporated in the organic group-containing metal oxide.

The hologram recording medium of the present invention comprises atleast the above-mentioned hologram recording material layer. Usually, ahologram recording medium comprises a supporting substrate (i.e., asubstrate) and a hologram recording material layer; however, a hologramrecording medium may be made only of a hologram recording material layerwithout having any supporting substrate. For example, a medium composedonly of a hologram recording material layer may be obtained by formingthe hologram recording material layer onto the substrate by application,and then peeling the hologram recording material layer off from thesubstrate. In this case, the hologram recording material layer is, forexample, a layer having a thickness in the order of millimeters.

The hologram recording medium of the present invention is suitable forrecord and reproduction using not only a green laser light but also ablue laser light having a wavelength of 350 to 450 nm. When thereproduction is made using transmitted light, the medium preferably hasa light transmittance of 50% or more at a wavelength of 405 nm. When thereproduction is made using reflected light, the medium preferably has alight reflectance of 25% or more at a wavelength of 405 nm.

The hologram recording medium is either of a medium having a structurefor performing reproduction using transmitted light (hereinafterreferred to as a transmitted light reproducing type medium), and amedium having a structure for performing reproduction using reflectedlight (hereinafter referred to as a reflected light reproducing typemedium) in accordance with an optical system used for the medium.

The transmitted light reproducing type medium is constructed in such amanner that a laser light for readout is irradiated into the medium, thelaser light irradiated therein is diffracted by signals recorded in itshologram recording material layer, and the laser light transmittedthrough the medium is converted to electric signals by means of an imagesensor. In other words, in the transmitted light reproducing typemedium, the laser light to be detected is transmitted through the mediumtoward the medium side opposite to the medium side into which thereproducing laser light is irradiated. The transmitted light reproducingtype medium usually has a structure wherein its recording material layeris sandwiched between two supporting substrates. In an optical systemused for the medium, the image sensor, for detecting the transmittedlaser light, is set up in the medium side opposite to the medium sideinto which the reproducing laser light emitted from a light source isirradiated.

Accordingly, in the transmitted light reproducing type medium, thesupporting substrate, the recording material layer, and any otheroptional layer(s) are each made of a light-transmitting material. It isunallowable that any element blocking the transmission of thereproducing laser light is substantially present. The supportingsubstrate is usually a rigid substrate made of glass or resin.

In the meantime, the reflected light reproducing type medium isconstructed in such a manner that a laser light for readout isirradiated into the medium, the laser light irradiated therein isdiffracted by signals recorded in its hologram recording material layer,and then, the laser light is reflected on its reflective film, and thereflected laser light is converted to electric signals by means of animage sensor. In other words, in the reflected light reproducing typemedium, the laser light to be detected is reflected toward the samemedium side as the medium side into which the reproducing laser light isirradiated. The reflected light reproducing type medium usually has astructure wherein the recording material layer is formed on a supportingsubstrate positioned at the medium side into which the reproducing laserlight is irradiated; and a reflective film and an another supportingsubstrate are formed on the recording material layer. In an opticalsystem used for the medium, the image sensor, for detecting thereflected laser light, is set up in the same medium side as the mediumside into which the reproducing laser light emitted from a light sourceis irradiated.

Accordingly, in the reflected light reproducing type medium, thesupporting substrate positioned at the medium surface side into whichthe reproducing laser light is irradiated, the recording material layer,and other optional layer(s) positioned nearer to the medium side intowhich the reproducing laser light is irradiated than the reflective filmare each made of a light-transmitting material. It is unallowable thatthese members each substantially contain an element blocking theincident or reflective reproducing laser light. The supporting substrateis usually a rigid substrate made of glass or resin. The supportingsubstrate positioned at the medium surface side into which thereproducing laser light is irradiated is required to have alight-transmitting property.

In any case of the transmitted light reproducing type medium and thereflected light reproducing type medium, it is important that thehologram recording material layer has a high light transmittance of, forexample, 50% or more at a wavelength of 405 nm. For example, in the caseof considering a layer (100 μm in thickness) composed only of the matrixmaterial (organic group-containing metal oxide material), it ispreferred that the layer has a high light transmittance of 90% or moreat a wavelength of 405 nm.

The hologram recording material layer obtained as above-mentioned has ahigh transmittance to a blue laser. Therefore, even if a thickness ofthe recording material layer is set to 100 μm, a recording medium havinga light transmittance of 50% or more, preferably 55% or more at awavelength of 405 nm is obtained when the medium is a transmitted lightreproducing type medium; or a recording medium having a lightreflectance of 25% or more, preferably 27.5% or more at a wavelength of405 nm is obtained when the medium is a reflected light reproducing typemedium. In order to attain holographic memory recording characteristicssuch that a high multiplicity is ensured, necessary is a recordingmaterial layer having a thickness of 100 μm or more, preferably 200 μmor more. According to the present invention, however, even if thethickness of the recording material layer is set to, for example, 1 mm,it is possible to ensure a light transmittance of 50% or more at awavelength of 405 nm (when the medium is a transmitted light reproducingtype medium), or a light reflectance of 25% or more at a wavelength of405 nm (when the medium is a reflected light reproducing type medium).

When the above described hologram recording material layer is used, ahologram recording medium having a recording layer thickness of 100 μmor more, which is suitable for data storage, can be obtained. Thehologram recording medium can be produced by forming the hologramrecording material in a film form onto a substrate, or sandwiching thehologram recording material in a film form between substrates.

In a transmitted light reproducing type medium, it is preferred to use,for the substrate(s), a material transparent to a recording/reproducingwavelength, such as glass or resin. It is preferred to form ananti-reflection film against the recording/reproducing wavelength forpreventing noises or give address signals and so on, onto the substratesurface at the side opposite to the layer of the hologram recordingmaterial. In order to prevent interface reflection, which results innoises, it is preferred that the refractive index of the hologramrecording material and that of the substrate are substantially equal toeach other. It is allowable to form, between the hologram recordingmaterial layer and the substrate, a refractive index adjusting layercomprising a resin material or oil material having a refractive indexsubstantially equal to that of the recording material or the substrate.In order to keep the thickness of the hologram recording material layerbetween the substrates, a spacer suitable for the thickness between thesubstrates may be arranged. End faces of the recording material mediumare preferably subjected to treatment for sealing the recordingmaterial.

About the reflected light reproducing type medium, it is preferred thatthe substrate positioned at the medium surface side into which areproducing laser light is irradiated is made of a material transparentto a recording and reproducing wavelength, such as glass or resin. Asthe substrate positioned at the medium surface side opposite to themedium surface side into which a reproducing laser light is irradiated,a substrate having thereon a reflective film is used. Specifically, areflective film made of, for example, Al, Ag, Au or an alloy made mainlyof these metals and the like is formed on a surface of a rigid substrate(which is not required to have a light-transmitting property), such asglass or resin, by vapor deposition, sputtering, ion plating, or anyother film-forming method, whereby a substrate having thereon thereflective film is obtained. A hologram recording material layer isprovided so as to have a predetermined thickness on the surface of thereflective film of this substrate, and further a light-transmittingsubstrate is caused to adhere onto the surface of this recordingmaterial layer. An adhesive layer, a flattening layer and the like maybe provided between the hologram recording material layer and thereflective film, and/or between the hologram recording material layerand the light-transmitting substrate. It is also unallowable that theseoptional layers hinder the transmission of the laser light. Others thanthis matter are the same as in the above-mentioned transmitted lightreproducing type medium.

According to the hologram recording medium of the present invention, therecording material layer is uniform; thus, a problem of light scatteringis not caused. Further, at the time of recording, the photopolymerizableorganic compound is turned into a polymer in the exposed portion; thepreferred organic group-containing metal oxide has the organometallicunit, wherein two aromatic groups are bonded directly to one Si atom, sothat the organic group-containing metal oxide is very good compatiblewith the polymer. Therefore, according to the hologram recording mediumof the present invention, the compatibility is sufficiently kept andproblems of light scattering and a fall in the transmittance are notcaused when or after recording is performed.

Accordingly, the hologram recording medium having the hologram recordingmaterial of the present invention can be preferably used not only in asystem wherein record and reproduction are made using a green laserlight but also in a system wherein record and reproduction are madeusing a blue laser light having a wavelength of 350 to 450 nm.

EXAMPLES

The present invention will be specifically described by way of thefollowing examples; however, the invention is not limited to theexamples.

Example 1 (Synthesis of a Matrix Material)

In 2.0 mL of a n-butanol solvent, 4.28 g of tetra-n-butoxyzirconium(Zr(O-nBu)₄, manufactured by Azmax Co., Ltd.) and 1.31 g of o-toluicacid, which will be abbreviated to “o-TA” hereinafter, (manufactured byTokyo Kasei Kogyo Co., Ltd.) were mixed with each other at a roomtemperature, and then the mixture was stirred for 30 minutes. The ratioby mole of o-TA/Zr(O-nBu)₄ was 1/1. To this reaction mixture was added2.6 g of diphenyldimethoxysilane (manufactured by Shin-Etsu ChemicalCo., Ltd.) to prepare a metal alkoxide solution. The ratio by mole ofSi/Zr was 1/1.

To the above-mentioned metal alkoxide solution was dropwise added anacid catalyst solution composed of 0.15 mL of water, 0.06 mL of a 2 Naqueous solution of hydrochloric acid, and 1 mL of ethanol at a roomtemperature while the alkoxide solution was stirred. The solution wascontinuously stirred for 30 minutes to conduct hydrolysis andcondensation reaction. In this way, a sol solution was obtained.

(Photopolymerizable Compound)

To 100 parts by weight of polyethylene glycol diacrylate (M-245,manufactured by Toagosei Co., Ltd.) as a photopolymerizable compoundwere added 3 parts by weight of a photopolymerization initiatorIRGACURE-907 (IRG-907, manufactured by Ciba Specialty Chemicals K.K.)and 0.15 part by weight of thioxanthen-9-one as a photosensitizer, so asto prepare a mixture containing the photopolymerizable compound.

(Hologram Recording Material)

The sol solution and the mixture containing the photopolymerizablecompound were mixed with each other at a room temperature to set theratio of the matrix material (as a nonvolatile component) and that ofthe photopolymerizable compound to 85 parts by weight and 15 parts byweight, respectively. Furthermore, the sol-gel reaction was sufficientlyadvanced for 1 hour in a state that light was shielded from the system,so as to yield a hologram recording material solution.

The resultant hologram recording material solution was applied onto aglass substrate and then dried to prepare a recording medium sample, aswill be detailed below.

With reference to FIG. 1, which schematically illustrates a crosssection of a hologram recording medium, explanation will be described.

A glass substrate (22) having a thickness of 1 mm and having one surfaceon which an anti-reflection film (22 a) was formed was prepared. Aspacer (24) having a predetermined thickness was put on a surface of theglass substrate (22) on which the anti-reflection film (22 a) was notformed, and the hologram recording material solution obtained wasapplied onto the surface of the glass substrate (22). The resultant wasdried at a room temperature for 1 hour, and then dried at 40° C. for 24hours to volatilize the solvent. Through this drying step, the gelation(condensation reaction) of the organic group-containing metal oxide wasadvanced so as to yield a hologram recording material layer (21) havinga dry film thickness of 310 μm wherein the organic group-containingmetal oxide and the photopolymerizable compound were uniformlydispersed.

(Hologram Recording Medium)

The hologram recording material layer (21) formed on the glass substrate(22) was covered with another glass substrate (23) having a thickness of1 mm and having one surface on which an anti-reflection film (23 a) wasformed. At this time, the covering was carried out in such a manner thata surface of the glass substrate (23) on which the anti-reflection film(23 a) was not formed would contact the surface of the hologramrecording material layer (21). In this way, a hologram recording medium(11) was obtained which had a structure wherein the hologram recordingmaterial layer (21) was sandwiched between the two glass substrates (22)and (23).

(Evaluation of Characteristics)

About the resultant hologram recording material sample, characteristicsthereof were evaluated in a hologram recording optical system asillustrated in FIG. 2. The direction along which the paper surface onwhich FIG. 2 is drawn stretches is defined as a horizontal direction forconvenience' sake.

In FIG. 2, the hologram recording medium sample (11) was set to make therecording material layer perpendicular to the horizontal direction.

In the hologram recording optical system illustrated in FIG. 2, a lightsource (101) for emitting a semiconductor laser (wavelength: 405 nm) ina single mode oscillation was used. Light emitted from this light source(101) was subjected to a spatial filtrating treatment by means of a beamrectifier (102), a light isolator (103), a shutter (104), a convex lens(105), a pinhole (106), and a convex lens (107), so as to be collimated,thereby enlarging the light into a beam diameter of about 10 mmφ. Theenlarged beam was passed through a mirror (108) and a ½ wavelength plate(109) to take out 45° (45 degree) polarized light. The light was splitinto an S wave and a P wave (the ratio of S wave/P wave is 1/1) througha polarized beam splitter (110). The S wave obtained by the splittingwas passed through a mirror (115), a polarizing filter (116), and aniris diaphragm (117) while a ½ wavelength plate (111) was used toconvert the P wave obtained by the splitting to an S wave and then the Swave was passed through a mirror (112), a polarizing filter (113) and aniris diaphragm (114). In this way, the total incident angle θ of the twolight fluxes irradiated into the hologram recording medium sample (11)was set to 37°, so as to record interference fringes of the two lightfluxes in the sample (11).

The sample (11) was rotated in the horizontal direction to attainmultiplexing (angle multiplexing; sample angle: −21° to +21°, angleinterval: 0.6°), thereby attaining hologram recording. The multiplicitywas 71. At the time of recording, the sample was exposed to the lightwhile the iris diaphragms (114) and (117) were each set to a diameter of4 mm. At a position where the angle of the surfaces of the sample (11)to the bisector (not illustrated) of the angle θ made by the two lightfluxes was 90°, the above-mentioned sample angle was set to ±0°.

After the hologram recording, in order to react remaining unreactedcomponents, a sufficient quantity of blue light having a wavelength of400 nm was irradiated to the whole of the surface of the sample (11)from a blue LED. At this time, the light was irradiated through anacrylic resin diffuser plate having a light transmittance of 80% so asto cause the irradiated light not to have coherency (the lightirradiation is called post-cure).

At the time of reproduction, with shading by the shutter (121), the irisdiaphragm (117) was set into a diameter of 1 mm and only one light fluxwas irradiated. The sample (11) was continuously rotated into thehorizontal direction from −23° to +23°. In the individual anglepositions, the diffraction efficiency was measured with a power meter(120). When a change in the volume (a recording shrinkage) or a changein the average refractive index of the recording material layer is notgenerated before and after the recording, the diffraction peak angle inthe horizontal direction at the time of the recording is consistent withthat at the time of the reproduction. Actually, however, a recordingshrinkage or a change in the average refractive index is generated;therefore, the diffraction peak angle in the horizontal direction at thetime of the reproduction is slightly different from the diffraction peakangle in the horizontal direction at the time of the recording. For thisreason, at the time of the reproduction, the angle in the horizontaldirection was continuously changed and then the diffraction efficiencywas calculated from the peak intensity when a diffraction peak made itsappearance. In FIG. 2, reference number (119) represents a power meternot used in this example.

At this time, a dynamic range M/# (the sum of the square roots of thediffraction efficiencies) was a high value of 20.1, which was aconverted value corresponding to the case that the thickness of thehologram recording material layer was converted to 1 mm. A lighttransmittance of the medium (recording layer thickness: 310 μm) beforethe recording exposure to light (i.e., at the initial stage) was 66.0%at 405 nm. A fall in the light transmittance of the medium at 405 nm(i.e., the recording wavelength) after the recording was not observed.

Example 2

A sol solution was obtained in the same manner as in Example 1 exceptthat 1.31 g of o-toluic acid was changed to 1.31 g of m-toluic acid,which will be abbreviated to “m-TA” hereinafter, (manufactured by TokyoKasei Kogyo Co., Ltd.) in the synthesis of a matrix material. The ratioby mole of m-TA/Zr(O-nBu)₄ was 1/1. The ratio by mole of Si/Zr was 1/1.

A hologram recording material solution was prepared in the same manneras in Example 1 except that the resultant sol solution was used. Theresultant hologram recording material solution was applied in the samemanner as in Example 1 to yield a hologram recording material layer (21)having a dry thickness of 320 μm. A hologram recording medium (11) wasobtained in this manner.

About the resultant hologram recording medium sample, characteristicsthereof were evaluated in the same manner as in Example 1. As a result,a dynamic range M/# was a high value of 23.0, which was a convertedvalue corresponding to the case that the thickness of the hologramrecording material layer was converted to 1 mm.

A light transmittance of the medium (recording layer thickness: 320 μm)before the recording exposure to light (i.e., at the initial stage) was60.0% at 405 nm. A fall in the light transmittance of the medium at 405nm (i.e., the recording wavelength) after the recording was notobserved.

Example 3

A sol solution was obtained in the same manner as in Example 1 exceptthat 1.96 g of o-toluic acid (o-TA) (manufactured by Tokyo Kasei KogyoCo., Ltd.) was used for 4.28 g of tetra-n-butoxyzirconium (Zr(O-nBu)₄)(manufactured by Azmax Co., Ltd.) in the synthesis of a matrix material.The ratio by mole of o-TA/Zr(O-nBu)₄ was 1.5/1. The ratio by mole ofSi/Zr was 1/1.

A hologram recording material solution was prepared in the same manneras in Example 1 except that the resultant sol solution was used. Theresultant hologram recording material solution was applied in the samemanner as in Example 1 to yield a hologram recording material layer (21)having a dry thickness of 320 μm. A hologram recording medium (11) wasobtained in this manner.

About the resultant hologram recording medium sample, characteristicsthereof were evaluated in the same manner as in Example 1. As a result,a dynamic range M/# was a high value of 20.0, which was a convertedvalue corresponding to the case that the thickness of the hologramrecording material layer was converted to 1 mm.

A light transmittance of the medium (recording layer thickness: 320 μm)before the recording exposure to light (i.e., at the initial stage) was66.0% at 405 nm. A fall in the light transmittance of the medium at 405nm (i.e., the recording wavelength) after the recording was notobserved.

Example 4

A sol solution was obtained in the same manner as in Example 1 exceptthat 2.61 g of o-toluic acid (o-TA) (manufactured by Tokyo Kasei KogyoCo., Ltd.) was used for 4.28 g of tetra-n-butoxyzirconium (Zr(O-nBu)₄)(manufactured by Azmax Co., Ltd.) in the synthesis of a matrix material.The ratio by mole of o-TA/Zr(O-nBu)₄ was 2/1. The ratio by mole of Si/Zrwas 1/1.

A hologram recording material solution was prepared in the same manneras in Example 1 except that the resultant sol solution was used. Theresultant hologram recording material solution was applied in the samemanner as in Example 1 to yield a hologram recording material layer (21)having a dry thickness of 270 μm. A hologram recording medium (11) wasobtained in this manner.

About the resultant hologram recording medium sample, characteristicsthereof were evaluated in the same manner as in Example 1. As a result,a dynamic range M/# was a remarkably high value of 55.6, which was aconverted value corresponding to the case that the thickness of thehologram recording material layer was converted to 1 mm.

A light transmittance of the medium (recording layer thickness: 270 μm)before the recording exposure to light (i.e., at the initial stage) was65.0% at 405 nm. A fall in the light transmittance of the medium at 405nm (i.e., the recording wavelength) after the recording was notobserved.

Comparative Example 1

In this Comparative Example, a Ti alkoxide compound (i.e., a decamer oftitanium butoxide represented by the following structural formula) wasused, without using Zr alkoxide compound.

C₄H₉—[OTi(OC₄H₉)₂]_(k)—OC₄H₉, wherein k=10.

Synthesis of a Matrix Material

In 6 mL of a tetrahydrofuran solvent, 7.8 g of diphenyldimethoxysilane(manufactured by Shin-Etsu Chemical Co., Ltd.) and 7.2 g of the titaniumbutoxide decamer (B-10, manufactured by Nippon Soda Co., Ltd.) weremixed with each other to prepare a metal alkoxide solution. The ratio bymole of Si/Ti was 1/1.

An acid catalyst solution composed of 0.9 mL of water, 0.36 mL of a 2 Naqueous solution of hydrochloric acid, and 3 mL of tetrahydrofuran wasdropwise added to the metal alkoxide solution at a room temperature withstirring. The stirring was continued for 1 hour to conduct hydrolysisand condensation reaction. In this way, a sol solution was obtained.

A hologram recording material solution was prepared in the same manneras in Example 1 except that the resultant sol solution was used. Theresultant hologram recording material solution was applied in the samemanner as in Example 1 to yield a hologram recording material layer (21)having a dry thickness of 390 μm. A hologram recording medium (11) wasobtained in this manner.

About the resultant hologram recording medium sample, characteristicsthereof were evaluated in the same manner as in Example 1. As a result,a dynamic range M/# was 11.4, which was a converted value correspondingto the case that the thickness of the hologram recording material layerwas converted to 1 mm. However, a light transmittance of the medium(recording layer thickness: 390 μm) before the recording exposure tolight (i.e., at the initial stage) was 31.0% at 405 nm. After therecording, the light transmittance of the medium at 405 nm (i.e., therecording wavelength) was a lowered value of 1%.

1. A hologram recording material comprising: a metal oxide containing atleast Si and Zr as metals, wherein an aromatic carboxylic acid compoundis coordinated to Zr; and a photopolymerizable compound.
 2. The hologramrecording material according to claim 1, wherein the aromatic carboxylicacid compound is toluic acid.
 3. The hologram recording materialaccording to claim 1, wherein the metal oxide has an organometallic unitin which two aromatic groups are directly bonded to Si.
 4. The hologramrecording material according to claim 1, which further comprises aphotopolymerization initiator.
 5. A process for producing a hologramrecording material comprising: a metal oxide containing at least Si andZr as metals, wherein an aromatic carboxylic acid compound iscoordinated to Zr; and a photopolymerizable compound, the processcomprising: mixing an alkoxide compound of Zr as a starting materialwith an aromatic carboxylic acid compound to modify the alkoxidecompound of Zr with the aromatic carboxylic acid compound; mixing thealkoxide compound of Zr modified with the aromatic carboxylic acidcompound with an alkoxide compound of Si; hydrolyzing the mixed alkoxidecompounds to yield a precursor of a metal oxide; incorporating aphotopolymerizable compound into the resultant system before, during orafter the hydrolysis; and advancing polycondensation reaction of themetal oxide precursor into which the photopolymerizable compound isincorporated.
 6. A hologram recording medium which has a hologramrecording layer comprising the hologram recording material according toclaim
 1. 7. The hologram recording medium according to claim 6, whereinrecord/reproduction of said hologram recording medium are performedusing a laser light having a wavelength of 350 to 450 nm.